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This is a guide showing how to replace the primary drive on a Saab 900 gearbox.



Why would you want to replace the primary drive?



The primary drive transfers the engines power from the flywheel through to the gearbox and the size of the gears used in the primary drive controls the engine speed v.s. wheel speed ratio. Saab labelled the gearboxes so you can see what the primary drive ratio is on the gearbox. They used numbers from 4 through to 8. With a type-4 primary drive the engine has to turn fast to make the wheels go slow. With a type-8 primary drive the engine turns slow and the wheels turn fast.



Saab made quite a few different primary ratios over the years. The primary drive ratio needs to match the engine performance. If you have a powerful T16 engine you need a type-7 or type-8 primary drive as the engine is powerful enough to move you easily with that ratio. If you have a base spec 8v carburettor model you need something like a type-6 primary drive or else the engine will struggle to move the car. The basic design of the box carried over from the Saab 99 and the same principles apply to the boxes used in those cars.



In the real world the times that primary drive is an issue is cruising on the motorway at 70 or 80 mph and when you’re accelerating through 1st and 2nd gear as you pull-away. The type-8 primary drive gives you a nice low RPM for motorway cruising, keeps the wheel-spin down a little and makes the gears last a slight bit longer during 1st and 2nd. When the wrong gearbox is mated to the wrong engine you get an odd miss-match where the wheels spin faster or slower than they should relative to the engine speed.



So that’s the conjecture out of the way and now I’m going to talk you through the method I used for changing from type-6 to type-8 primaries. Usual disclaimer of follow this guide at your own risk and don’t come running to me after you’ve dropped a gearbox on your feet. I can’t be responsible for any injuries you might cause yourself. I suggest you read the guide the whole way through before you start so you know the process. Hopefully everything in the guide is accurate but please to correct if you spot a mistake.





You can replace the primary drive with the engine in the car but you would need to remove the front bumper, headlights and radiator to get to clearance to operate at the front of the gearbox. In my scenario I’m also replacing the gearbox so I’ve got the engine out in any case. The transmission is resting on an old chair.





1. Loosen the 12mm bolts securing the cover at the front of the gearbox. I tend to find a good sharp ‘crack’ with a breaker bar is my preferred method for loosening old fasteners. Slow and imprecise spanner work can round-off the head on the bolt and then you’re a bit stuck so try to avoid that.






2. Use a screwdriver or chisel to prise the cover off the front of the gearbox. You shouldn’t need much force and you could yank it off by hand but then the oil would spill everywhere and it’s better if that doesn’t happen.





3. Use an old container to catch the oil as it drips out the bottom of the primary drive case. The gearbox contains about 3 litres in total but only ½ a litre of that will leak out at this point. Be careful though as used gearbox oil has a nasty smell and you don’t want it spilling everywhere.




4. Take the cover off the primary drive case and put it down in such a way that it doesn’t get dirty. i.e. not on the concrete floor. You need to keep the gearbox internals completely grit-free.





5. Here’s how it looks under the cover. The sprocket at the top accepts the power from the engine and transmits it through to the lower sprocket which feeds through to the gearset. There is a chain tensioner in the middle pushing the chains outward to keep them taut.

 

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Discussion Starter #2
6. Remove the tensioner.







The tensioner. I kept all the important parts on a clean grit-free surface.








7. The upper sprocket is fixed to the gearbox with a circlip.




You might have a pair of circlip pliers you can use to remove the circlip but I don’t have that luxury so I used two spikes on either side of a spoke to stretch the circlip open. Then I used a screwdriver to hold the clip open and moved the spikes to the other side of the circlip and forced it off the shaft.







The arrows point to the loose circlip. It will just float around inside the sprocket as it’s trapped between the sprocket and the bearing. Leave it in there as you need to reuse it when you re-fit the sprocket.


 

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8. Next you need to remove the nut securing the lower sprocket. It’s a 32mm nut. I had to buy a socket to fit this as I didn’t already have one. Notice the flange on the bottom of the nut is pushed into the holes to stop it turning. This makes it quite difficult to remove as you have to break the metal to move the nut. To date I have not needed more force on any other nut I’ve removed. If you have air tools get them out now – it’s a tough nut.















9. Remove the chains and sprockets from the gearbox case. The lower sprocket slips straight off but the upper sprocket needs to come off evenly. If you try to pull it from just one side it will get stuck and won’t come off. Clean out any dirt or grit if there is any.



 

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10. So I repeated the procedure on the broken type-8 box to rescue the primary drive before taking the old broken gearbox to the waste disposal site.



Side-by-side you can see a clear difference between the type-8’s on the left and the type-6’s on the right. Notice I kept the type-8 chains and sprockets as one complete unit. I did this so the chains and sprockets went back on the same way they came off. This might be a bit paranoid but I reason that the chains and sprockets would be worn and shouldn’t be mixed up – if the parts were new it wouldn’t matter but the old parts should keep turning the same way they always have.






11. Fit the type-8 primaries to the box. Make sure you don’t get any dirt or grit on any of the internal parts.






12. The lower sprocket nut had threadlock on it originally so I made sure I threadlocked again when refitting the nut. Quite a liberal application.





13. Re-fit the nut and tighten as much as you can. Don’t go too crazy though. Something might snap. Just make it fairly tight. If anyone has the Saab literature explaining how tight this nut should be then please contribute.





14. Use a nail-punch (or similar tool) to punch the flange back into the holes on the sprocket. This nut now has two redundant marks where it was originally punched at the factory and three new marks where I’ve re-punched it.

 

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15. Re-fit the circlip to secure the top sprocket. I used long-nose pliers to open up the circlip while I pushed it back onto the shaft.



16. Clean the old thread-lock off the tensioner bolts and apply some fresh threadlock. Quite liberal again here. I’d imagine these bolts get shaken around a fair bit so it’s a good place for a decent bit of threadlock. Don’t tighten too much here. 15 ft-lb is the torque setting. Too tight and you’ll bust the threads on the aluminium gearbox case! - not pretty!










17. If you have a water cooled turbo then there will be two studs on screwed into the primary drive case. I used threadlock on these because they’re difficult to tighten.




18. Good old Loctite 518 anaerobic sealant. This is the same as Saab part number: 9321795 and should be used on joins between aluminium parts. It does not blob and cure like silicone. It won’t block the oil-ways if it seeps out of the joint and falls into the lubrication system like silicone can.

 

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Discussion Starter #6
19. Clean both mating surfaces and make sure there is no grit or dirt anywhere inside the primary drive case or on the mating surfaces.



Apply the sealant. The old paper gasket had two parallel strips and I’ve replicated that with my application of the liquid sealant. If you’re a cheapskate you could probably getaway with a single circuit but I really don’t want the oil to leave the gearbox so I followed the original line. Make it a complete line - don’t leave any gaps or the seal may not work.






20. Re-fit the cover and move it around a bit so the sealant is smeared around between the mating surfaces.





21. Re-install the fasteners. Tighten to 15 ft-lb.





22. Pay special attention to the cleanliness of this seal. A few bits of grit here can cause quite a substantial leak from the front of the box. This pic shows the dirt on it before I cleaned it.





That's it. :cool:
 

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Great writeup, and a first AFAIK - never seen a primary drive replacement how-to before :cool: If I buy a certain tidy 900 with type 6 primaries, then I may need to do this job.

Going straight into the FAQ :cool:

ejenner said:
Notice the flange on the bottom of the nut is pushed into the holes to stop it turning. This makes it quite difficult to remove as you have to break the metal to move the nut.
The lower sprocket's nut looks like it's one-use only. In that case, to make it easier to remove, the flange could be prised away slightly from the sprocket's face.

The Saab service manual suggests to "use a drift to remove the peened-over material from the flange of the input shaft nut".

As for refitting the old inout shaft nut, well yes, they're £11.95 from Saab :eek: (part number 87 12 770) but this a job you do just the once. The original nut looked sufficiently trashed that I wouldn't be trusting it, even with new peening.

Interestingly the Saab service manual does not explicitly say to use a new nut.

ejenner said:
I reason that the chains and sprockets would be worn and shouldn’t be mixed up
Good reasoning and I agree. However, the type 6 and 8 sprockets and chains couldn't be mixed even if you wanted to, as the chains will be different lengths to accommodate the larger/smaller sprockets.
ejenner said:
If anyone has the Saab literature explaining how tight this nut should be then please contribute.
The Saab service manual for the transmission says 100Nm +/- 10Nm (74lb/ft +/- 7.4lb/ft).

I think it would be worth scrubbing the chain cover clean before refitting it.

ejenner said:
obviously as you've drained 1/2 a litre of the oil you'll need to drain out the rest of it and replace with fresh
Don't forget to pour 0.3 litres of oil into the gear case too.

What's the metalwork to the left of the lower sprocket drive in this photo?


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It can be done in the car; all you have to do is remove is the radiator, and support the motor. You can even tilt it up some.

If you put a big rubber band around the tensioner, you can leave it in place.
 

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grif900 said:
i think the lower nut can be "staked" as many times as needed as long as its correctly tourqued.
New nut is required according to SAAB; since that particular bearing preload is adjusted by shims, tighten the nut all you want.
The little metal trough collects oil that is slung off the chain and drains it into a cavity between the tapered bearings on the shaft; the outer tapered bearing is what you see. This provides a positive oil flow instead of just splash.
 

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Discussion Starter #13
Matthew said:
However, the type 6 and 8 sprockets and chains couldn't be mixed even if you wanted to, as the chains will be different lengths to accommodate the larger/smaller sprockets.
I used the chains from the type-8 box on the type-8 sprockets and I kept them as one complete unit without slipping any teeth or removing any chains. It's not difficult to do as both sprockets come off at the same time.

Matthew said:
The lower sprocket's nut looks like it's one-use only. In that case, to make it easier to remove, the flange could be prised away slightly from the sprocket's face.
I did try that but the angles don't make it easy so I PM'd boxman and nutcase to see how they did it and it seems brute-force is the answer.

Matthew said:
I think it would be worth scrubbing the chain cover clean before refitting it.
There's a good chance this could create more mess and grit particles on the inside of the cover than if you left it dirty. If you wanted to clean it up a bit the cleaning should be done before the cover is first removed or after the cover has been reattached.
 

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Discussion Starter #14 (Edited)
Jim Mesthene said:
If you put a big rubber band around the tensioner, you can leave it in place.
Good tip...

Jim Mesthene said:
It can be done in the car
ejenner said:
You can replace the primary drive with the engine in the car but you would need to remove the front bumper, headlights and radiator to get to clearance to operate at the front of the gearbox. In my scenario I’m also replacing the gearbox so I’ve got the engine out in any case. The transmission is resting on an old chair.
 

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ejenner said:
Good tip...
I didn't remove the headlights or bumper when I did the one at Nick's. However, I didn't have to put a set back :D

Agreed no need to remove the tensioner. Chains are all the same I believe, the tensioner just takes up any slack. Agreed keep assemblies together though if they're used.
 

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You'd get a type 5.5 ( (4+7) / 2 ).

The total number of teeth in each gearset (4 - 8) is the same. as you go higher, you take one tooth off the lower gear and put it on the upper gear so the chain length should be the same.
 

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Looking on 900aero.com, the type 8 has both the largest top gear, and the smallest bottom gear, for a manual box. But, the auto type 3 has 40 teeth on the top gear :).
Going on the same gaps as between the other types, to make a type 9, you could use 40 teeth on the top gear, and the bottom gear from a type 5 or 6 box. You'd probably need an extra link or two on the chain though :)
 
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